Devices intended for ascertaining whether or not a railroad switch is open are quite old and have been used in association with most switch mechanisms for many years in railroad yards, hump yards and on mainlines. Such mechanisms depend mostly upon the accuracy and capability of the switch itself to detect failures that will cause the switch point to open. Many devices are very sophisticated and electronically controlled, and such prior art devices frequently are intended to permit ascertaining whether or not the switch is closed from remote points, such as in a yard tower or many miles downline.
However, even in the light of such sophisticated equipment, railroads continue to suffer derailments. It is believed that the cause for many of these derailments can be traced to conventional mechanisms for detecting whether a switch is open or closed. Such prior art mechanisms are usually of a type which detect an obstructed open switch point, but only if the obstruction results in mechanical pressure directed against the switch machine. If there is no obstruction, the detector mechanism may cause an erroneous reading indicating that the switch is open or closed, when in fact the opposite is true.
Many more switches in the railway industry in the United States are manually operated than electrically powered. Such manually operated switch are frequently as dangerous as powered switches because they are usually located in areas where trains loaded with hazardous materials and chemicals and other critical cargos travel. Frequently, derailments at the locations of such switches are also caused by open switchpoints, usually caused by human error or failure to notice that the switchpoint is safely closed against the stockrail during operations in bad weather or without visibility. Also, such manually operated switches are usually in locations where service of the switches is not as effective as desired, making it easier to miss mistakes in calibrations or settings which lead to a false belief that a switchpoint is closed when in fact it is open.
Frequently, existing equipment is relied upon to perform functions not intended; for example, stop signs and similar devices are frequently mechanically attached to manually operated switch gear boxes, but these devices are designed only to indicate which route the switch is directed to and not to whether or not the switchpoint is open or closed. False reliance on these devices, frequently coupled with a switchman's view (usually under pressure to throw another switch and keep the train moving) leads the switchman to assume that the switchpoint is in the position designated just because the switch lever has dropped inside the switch lever slot (with no visual inspection of the switchpoint).
Even with powered switches located in hump or switch yards, where some of the most sophisticated equipment in the world is located, derailments are not infrequent and are often disastrous when they occur. Investigation after such events sometimes reveals an obstruction between the switchpoint and the stockrail, or misadjustment or worn out movable parts and mechanical components, sometimes even a simple light bulb. Such failures can be traced to loose or worn out movable parts and mechanical components, or poor maintenance or loose ballast conditions, or poor maintenance or loose stockrail braces, or worn out or loose throwrod adjusting nuts, or worn out or defective throw rod baskets, or broken throwrods, or worn out or broken lugs connecting the switch sectors and throwrods.
In other cases, the cause is human error, where the complaints that the sun light was too bright to see the safety light in the field or his view of a detector was obstructed. In these situations the "clear" safety signal indication, either in the tower or on the field, is false and can be the direct cause of a derailment. Operating personnel are sometime unable to see the condition of the switchpoint in time to take action avoiding a derailment, because a light is burned out or there is glare or darkened conditions or obstructions or for some other reason which prevents the operator from seeing whether the point is open or closed.
In any of these events, the failure can be traced to the fact that prior art detectors are adjusted and set according to calibration of the detector and the switch point, rather than a positive relationship between the stockrail and the switchrail at the point where they are intended to come together or be separated. In such failures, even if the conventional detector is initially properly calibrated and can be detected, changes occur which make the calibration inaccurate or obscure the detector. If the calibration is changed by some condition which is not regularly detected, sometimes by a misalignment of merely about 0.05 of an inch, such as movement of the tracks relative to one another or movement of the detector or switch components, which may result from vibrations of a locomotive or rolling stock over the track or by change in weather conditions, a derailment may result. Power may also be interrupted or a warning source obliterated or inoperative-any of which can cause a derailment.